Large numbers of Chinese citizens – including children – have been held for days or months in unofficial "black jails" that appear to have emerged when a controversial detention system was abolished, according to a report published today by a human rights group.

Dozens of citizens who had travelled or tried to travel to Beijing to seek redress for local injustices told Human Rights Watch they were instead abducted, detained and in many cases abused in the illicit prisons.

The prison issue received unusual coverage in the domestic media this year when a guard was accused of raping a young detainee – although the carefully worded articles, which did not include the term "black jails", were soon deleted from Chinese websites. The English-language newspaper China Daily reported last week that the guard had pleaded guilty and a verdict was expected within the month.

Foreign and domestic journalists, Chinese human rights organisations and Chinese scholars have collected details of several jails as well as speaking to those who have been held there.

But the Chinese government denies such sites exist. During a United Nations human rights commission review in June, it said: "There are no black jails in the country." Former detainees have said some sites were described as "legal education centres".

Human Rights Watch interviewed 38 former detainees for the report, An Alleyway in Hell, but withheld all names for fear of reprisals. Interviewees reported physical violence by guards, extortion, threats and deprivation of food, sleep and medical care.

"Two people dragged me by the hair and put me into the car … They put me inside a room where there were two women who stripped me of my clothes … beat my head [and] used their feet to stomp my body," said a woman from Jiangsu province.

Another, from Sichuan, said her guards warned that if she tried to escape they would take her to the male prison and allow the inmates to rape her.

A 15-year-old girl told Human Rights Watch she was abducted while petitioning for her disabled father, locked up for more than two months in Gansu province and severely beaten.

Tens of thousands of petitioners head to Beijing each year in a last-ditch attempt to solve problems in their home towns such as land grabs or corruption. A law professor, He Weifang, is cited in the report as comparing the system to "drinking poison to quench a thirst".

Local governments try to stop citizens seeking redress from the central government because officials' careers suffer if many petitions are filed from their area. The report says the operators of black jails receive fees for each person held, creating another incentive for detention.

It says the jails appear to have emerged since the custody-and-repatriation system – which allowed police to return those without official permission to live in cities to their home towns – was abolished in 2003, following a national outcry over the death of a young man in one centre.

"The existence of black jails in the heart of Beijing makes a mockery of the Chinese government's rhetoric on improving human rights and respecting the rule of law," said Sophie Richardson, Asia advocacy director for Human Rights Watch.

She said China was ignoring its own laws and urged the government to close the facilities, investigate those running them and provide help to those abused.